Measurement and Modeling of Polarized Atmosphere at the South Pole with SPT-3G

TL;DR

This study characterizes polarized atmospheric fluctuations at the South Pole using SPT-3G data, modeling their origin and impact on CMB polarization measurements, and proposing mitigation strategies for future observations.

Contribution

First detailed measurement and modeling of polarized atmospheric emission at the South Pole, including mitigation approaches for CMB polarization experiments.

Findings

Polarized atmospheric emission is consistent with Rayleigh scattering and ice crystal emission.

The spatial and frequency scaling of polarization matches a Kolmogorov turbulence model.

Mitigation strategies effectively reduce atmospheric polarization impact in SPT-3G data.

Abstract

We present the detection and characterization of fluctuations in linearly polarized emission from the atmosphere above the South Pole. These measurements make use of data from the SPT-3G receiver on the South Pole Telescope in three frequency bands centered at 95, 150, and 220 GHz. We use the cross-correlation between detectors to produce an unbiased estimate of the power in Stokes I, Q, and U parameters on large angular scales. Our results are consistent with the polarized signal being produced by the combination of Rayleigh scattering of thermal radiation from the ground and thermal emission from a population of horizontally aligned ice crystals with an anisotropic distribution described by Kolmogorov turbulence. The measured spatial scaling, frequency scaling, and elevation dependence of the polarized emission are explained by this model. Polarized atmospheric emission has the potential to significantly impact observations on the large angular scales being targeted by searches for inflationary B-mode CMB polarization. We present the distribution of measured angular power spectrum amplitudes in Stokes Q and I for 4 yr of Austral winter observations, which can be used to simulate the impact of atmospheric polarization and intensity fluctuations at the South Pole on a specified experiment and observation strategy. We present a mitigation strategy that involves both downweighting significantly contaminated observations and subtracting a polarized atmospheric signal from the 150 GHz band maps. In observations with the SPT-3G instrument, the polarized atmospheric signal is a well-understood and subdominant contribution to the measured noise after implementing the mitigation strategies described here.